CN111272881A - Laser ultrasonic system and method for detecting thermal diffusivity of nano film in non-contact mode - Google Patents
Laser ultrasonic system and method for detecting thermal diffusivity of nano film in non-contact mode Download PDFInfo
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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
Technical Field
The invention belongs to the field of laser ultrasonic non-contact detection, and particularly relates to a laser ultrasonic system and a laser ultrasonic method for detecting thermal diffusivity of a nano film in a non-contact manner.
Background
The nano film material has the characteristics of impact resistance, wear resistance, thermal breakdown resistance and the like. They are commonly used as conductive and dielectric elements in microelectronics, as reflective or polarizing elements in integrated optics, and in many other applications in the aerospace, biotechnology, automotive, photographic and magnetic disk industries. Many conventional spectroscopic and scanning detection techniques can determine the chemical, morphological, etc. properties of the nanofilm. However, suitable techniques for measuring the thermal diffusivity of thin films are not numerous, in part because nanomaterials can be thin or small and are easily damaged.
On the premise of not damaging various performances of a sample to be detected, the laser ultrasonic nondestructive detection can detect related physical properties or structural characteristics of the sample in time, such as sound velocity in a detection substance, thermal diffusion coefficient and mass diffusion coefficient in liquid or solid substances, temperature measurement of gas substances, carrier bipolar diffusion coefficient and non-equilibrium carrier service life of semiconductor materials, carrier mobility and the like. Due to the characteristics of no damage, rapidness, accuracy and the like, the photoacoustic detection technology is applied to the measurement of the thermal diffusivity of the nano film material.
The excited sample surface generates micro-vibration caused by surface ultrasound, and the excited part is periodically distributed in an undulating manner. And because the pulse laser is used for excitation, the excited micro-vibration is unstable transient grating. The probe/reference light is incident on the surface of the sample, reflected or diffracted by the sample, and finally received by a signal receiving module of the system. The change rule of the heterodyne signal intensity diffracted by the sound wave along with the time is analyzed, and the relation between the detected signal and the thermal diffusivity is obtained.
However, it is difficult to obtain the thermal diffusivity of the film 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 diffraction light intensity information may be very weak, and it is not enough to analyze the related information of the ultrasonic signal, so it is necessary to combine the heterodyne detection technology. The optical heterodyne technique couples a detection optical signal directly parametered with reference light, and a detector obtains the coupled signal, and the signal size of the coupled signal can be tens of times of that of a signal detected by a direct detection method, so that an optical heterodyne detection method is very sensitive to micro-vibration. But adjusting the phase relationship between the two beams to achieve the coupling requirement is difficult. A conventional heterodyne system uses rotation of a glass plate to control the optical path length of one of the beams, thereby controlling the phase difference between the two beams. However, this method is rough and cannot accurately meet the requirement of precise phase matching between two beams.
In summary, although the photoacoustic nondestructive testing technique can nondestructively, rapidly, and accurately test the thermal diffusivity of a thin film. 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 very small, which is not enough to analyze the related information of the ultrasonic signal, and the thermal diffusivity of the film cannot be analyzed. If a common heterodyne detection system with parallel plates is added, the adjustment mode is rough and not accurate enough, the requirement of accurate phase matching between two light beams cannot be accurately finished, and the accurate measurement of the thermal diffusivity of the film cannot be realized.
Disclosure of Invention
The invention aims to provide a laser ultrasonic system for detecting the thermal diffusivity of a nano film in a non-contact manner.
In order to achieve the purpose, the invention adopts the following technical scheme: the utility model provides a non-contact detects laser ultrasonic system of nanometer film thermal diffusivity, includes exciting light emission module, detects light emission module, dichroic mirror, phase place mask, photochopper, first achromat lens, phase regulator, second achromat lens, removal sample platform and signal receiving module, and is set up to:
exciting light emitted by the exciting light emitting module is transmitted by the dichroic mirror and then focused on the phase mask plate, the phase mask plate divides the exciting light into two beams, and the two beams of exciting light are sequentially focused on the surface of a sample arranged on the movable sample stage through the light chopper, the first achromatic lens and the second achromatic lens;
the detection light emitted by the detection light emitting module is reflected by the dichroic mirror and then focused on the phase mask plate, the phase mask plate divides the detection light into two beams, one beam of the detection light is used as detection light and sequentially focused on the surface of a sample arranged on the movable sample stage through the light chopper, the first achromatic lens, the phase adjuster and the second achromatic lens, and the other beam of the detection light is used as reference light and sequentially focused on the surface of the sample arranged on the movable sample stage through the light chopper, the first achromatic lens and the second achromatic lens;
the two excitation lights are overlapped with the detection light and the reference light at the same position of the sample;
the signal receiving module is used for receiving heterodyne signals after the reflection and the diffraction of the sample.
Preferably, the light emitting module includes a 532nm pulse laser, an attenuation sheet and a cylindrical mirror, the 532nm pulse laser, the attenuation sheet and the cylindrical mirror are located on the same optical axis, and the cylindrical mirror focuses the excitation light onto the phase mask.
Preferably, the detection light emitting module includes an 830nm continuous laser and a spherical lens, and the 830nm continuous laser and the spherical lens are located on the same optical axis.
Preferably, the signal receiving module comprises a focusing sphere lens and an optoelectronic signal receiver:
the focusing sphere lens is used for coupling the detection light carrying the sample information excited by the exciting light and the reference light into the photoelectric detector to obtain a 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 of the phase mask, and the detection light and the excitation light form a fixed angle, so that the phase adjuster only allows the detection light to pass through without shielding the excitation light.
Preferably, the heterodyne signal received by the signal receiving module is:
wherein q is the wavelength of the excited sound wave, ISFor detecting the intensity of light, I, after reflection by the sampleRLambda is the wavelength of the excitation light, and u (t) is the micro-vibration correlation function of the sample surface.
Preferably, the sample surface microvibration correlation function is specifically:
u(t)＝U0sin(qx)
in the formula of U0Is the amplitude of the ultrasonic displacement, q is the wavelength of the excited sound wave;
the reference light intensity after sample diffraction is specifically as follows:
in the formula I0The intensity of the reference light without diffraction by the sample.
The invention also provides a method for detecting the thermal diffusivity of the nano film in a non-contact way, which comprises the following steps:
constructing a laser ultrasonic system;
obtaining heterodyne signals after sample reflection and diffraction;
and calculating according to the heterodyne signals to obtain the thermal expansion rate of the sample.
Preferably, the specific method for obtaining the sample thermal expansion rate by calculating according to the heterodyne signal is as follows:
will ultrasonic displacement amplitude U0Substituting the expression of (3) and the expression of the acoustic wave length into the heterodyne signal to obtain the thermal diffusivity D of the film, wherein the ultrasonic displacement amplitude U0The expression is as follows:
wherein d is a linear thermal expansion coefficient, E0The average energy density of the exciting light is shown, rho is the material density, c is the specific heat, and D is the thermal diffusion coefficient;
the expression of the excited wave length q is as follows:
wherein theta is a diffraction angle obtained after the excitation light is diffracted by the phase mask plate, lambda is the wavelength of the excitation light, and lambda is a grating constant of the thermal grating generated by exciting the sample by the excitation light;
preferably, the thermal diffusivity obtained is:
wherein τ is a relaxation time at the wavelength of the excited acoustic wave, and Λ is a relaxation time at the wavelength of the excited acoustic wave.
Compared with the prior art, the invention has the following remarkable advantages:
based on the principle of photoacoustic excitation and detection, the photoacoustic nondestructive detection technology is combined with the heterodyne detection technology, and a phase regulator and other related elements are added into the system, so that the detected photoacoustic signal is stronger, the contained related information is more accurate, and the thermal diffusivity of the analyzed nano film is more accurate; the invention can realize the nondestructive, accurate and rapid measurement of the nano film material under the condition of not contacting with a sample.
Drawings
FIG. 1 is a schematic structural diagram of a laser ultrasonic system for non-contact detection of thermal diffusivity of a nano-film.
FIG. 2 is a schematic top view of a partial structure of a laser ultrasonic system for non-contact detection of thermal diffusivity of a nano-film in accordance with the present invention.
FIG. 3 is a diagram illustrating the simulation of the focal spots of the excitation light and the probe/reference light focused on the sample according to the present invention.
FIG. 4 is a schematic diagram showing the reflection and diffraction of probe light and excitation light, respectively, after passing through a thin film excited by the excitation light.
Detailed Description
As shown in fig. 1 and 2, a laser ultrasonic system for non-contact detection of thermal diffusivity of a nano-film comprises an excitation light emitting module, a detection light emitting module, a dichroic mirror 4, a phase mask 7, a light chopper 8, a first achromatic lens 9, a phase adjuster 10, a second achromatic lens 11, a mobile sample stage and a signal receiving module, and is configured to:
exciting light emitted by the exciting light emitting module is transmitted by the dichroic mirror 4 and then focused on the phase mask plate 7, the exciting light is divided into two beams by the phase mask plate 7, and the two beams of exciting light are sequentially focused on the surface of a sample arranged on the movable sample stage through the light chopper 8, the first achromatic lens 9 and the second achromatic lens 11;
the detection light emitted by the detection light emitting module is reflected by the dichroic mirror 4 and then focused on the phase mask plate 7, the phase mask plate 7 divides the detection light into two beams, one beam of the detection light is used as detection light and sequentially focused on the surface of a sample arranged on the movable sample stage through the light chopper 8, the first achromatic lens 9, the phase adjuster 10 and the second achromatic lens 11, and the other beam of the detection light is used as reference light and sequentially focused on the surface of the sample arranged on the movable sample stage through the light chopper 8, the first achromatic lens 9 and the second achromatic lens 11;
the two excitation lights are overlapped with the detection light and the reference light at the same position of the sample, so that the requirements on the size and the energy of the light spot are met.
The signal receiving module is used for receiving heterodyne signals after the reflection and the diffraction of the sample.
Specifically, the phase mask 7 is located on a focal plane of the cylindrical mirror, when the excitation light is transmitted by 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 light with different energies, the diffraction light has different grating periods, and a sample can be detected by using different grating periods for multiple times, so that heterodyne signals under different grating periods can be obtained.
The light shield 8 is used to shield diffracted light of each level, which is not used to excite a sound wave after the 532nm laser light is diffracted by the phase mask 7.
Specifically, the first achromatic lens 9 and the second achromatic lens 11 are configured to focus excitation light diffracted by the phase mask 7 and used for exciting a surface acoustic wave of the sample onto the surface of the sample, so that two excitation lights are spatially and temporally overlapped on the surface of the sample to form interference fringes and generate surface ultrasound.
In a further embodiment, the light emitting module includes a 532nm pulse laser 1, an attenuation sheet 2, and a cylindrical mirror 3, where the 532nm pulse laser 1, the attenuation sheet 2, and the cylindrical mirror 3 are located on the same optical axis, and the cylindrical mirror 3 focuses the excitation light onto the phase mask 7.
Specifically, the 532nm pulse laser 1 is used to generate excitation light.
The attenuation sheet 2 is used for adjusting the energy of laser emitted by the 532nm pulse laser to excite an acoustic wave signal.
The cylindrical mirror 3 is used for focusing the excitation light on the phase mask plate 7, and the height of the excitation light in the vertical direction is the same as that of the laser light outlet hole.
In a further embodiment, the detection light emitting module includes an 830nm continuous laser 5 and a spherical lens 6, and the 830nm continuous laser 5 and the spherical lens 6 are located on the same optical axis.
Specifically, the 830nm 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 different levels of diffraction light with different energies, the diffraction light has different grating periods, and a sample can be detected by using different grating periods for multiple times, so that heterodyne signals under different grating periods can be obtained.
Specifically, the phase adjuster 10 is an electro-optical phase modulator based on an electro-optical effect, applies a voltage to an input terminal electrode to generate a phase difference controlled by the voltage, but does not affect a polarization direction, can adjust a laser beam to a desired state by adjusting the voltage, can accurately and dynamically adjust a phase condition of incident light, and optimizes a heterodyne phase to finally detect an ultrasonic micro-vibration condition, where a detection light and a reference light after the phase adjustment reach a phase matching condition.
In a further embodiment, the signal receiving module includes a focusing sphere lens 13 and an optoelectronic signal receiver 14:
the focusing sphere lens 13 is used for coupling the detection light carrying the sample information excited by the excitation light and the reference light into the photoelectric detector 14 to obtain a heterodyne signal.
In particular, the moving sample stage is a three-dimensional motorized translation stage 12 for moving the sample to a 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 on the same point of the phase mask 7, and the detection light and the excitation light form a fixed angle, so that the phase adjuster 10 only allows the detection light to pass through without blocking the excitation light.
Specifically, the angle of the detection light to the excitation light is 7 °.
In a further embodiment, the heterodyne signal received by the signal receiving module is:
wherein q is the wavelength of the excited sound wave, ISFor detecting the intensity of light, I, after reflection by the sampleRLambda is the wavelength of the excitation light, and u (t) is the micro-vibration correlation function of the sample surface.
In a further embodiment, the sample surface microvibration correlation function is specifically:
u(t)＝U0sin(qx)
in the formula of U0Is the amplitude of the ultrasonic displacement, q is the wavelength of the excited sound wave;
the reference light intensity after sample diffraction is specifically as follows:
in the formula I0The intensity of the reference light without diffraction by the sample.
Based on the principles 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, the contained related information is more accurate, and the thermal diffusivity of the analyzed nano film is more accurate. And when the optical path is optimized, the position, the angle and the like of the related elements are adjusted, so that the related elements are prevented from blocking light. Finally, the invention can realize the nondestructive, accurate and rapid measurement of the nano film material under the condition of not contacting with a sample
A method for detecting thermal diffusivity of a nano-film in a non-contact manner specifically comprises the following steps:
constructing a laser ultrasonic system;
in some embodiments, the specific steps of constructing the laser ultrasound system are:
step 1, sequentially installing a 532nm pulse laser, an attenuation sheet, a cylindrical mirror, a dichroic mirror, a phase mask plate, a light chopper, a first achromatic lens 9 and a second achromatic lens 11 on a bottom plate. The 532nm pulse laser light-emitting hole, the attenuation sheet, the cylindrical mirror and the phase mask plate are all positioned on the same horizontal light path, and the light chopper and the achromatic lens are positioned on the same light path.
The height of the center of each element in the direction perpendicular to the fixed base plate is the same, and is 68.5 mm.
The reason why the cylindrical mirror is selected instead of the spherical mirror is that the cylindrical mirror collects energy with a larger density than the spherical mirror and the amplitude of the excited sound wave is larger under the condition of the same focal spot width. The time-domain waveform of the sound wave excited by the laser and the directivity of the sound wave are related to the spatial distribution of the focused spot. The light spot is focused by the cylindrical lens, the excited surface acoustic wave is bipolar, and compared with the point light spot focused by the spherical lens, the surface acoustic wave excited by the linear light plate focused by the cylindrical lens has stronger directivity.
And 2, setting a phase mask plate period.
The phase mask is first placed over the minimum grating period. Since the diffraction angle is the largest when the period is the smallest, the optical element size is guaranteed to meet the maximum requirements. For example, if the minimum period is 4 μm, and the probe light and the reference light are selected to be 0-order and-2-order diffracted lights of 830nm laser light, respectively, the maximum diffraction angle is calculated to be 11.97 °. Whereas the diffraction angle of the probe/reference light changes due to the changing grating period. The period is increased, and the two beams of the detection light/the reference light are converged to the center of the system.
And 3, adjusting the angle of the dichroic mirror.
The dichroic mirror is fixed at 48.5 degrees in the horizontal direction, the angle between the exciting light and the detection light is 7 degrees, and the spatial separation of the exciting light and the detection light/reference light is ensured.
The dichroic mirror is fixed at 11.97 degrees in the vertical direction, the angle is obtained according to the condition that the period of a phase mask plate is 4 mu m, and the selected probe light and the reference light are 0-order and-2-order diffraction light of 830nm laser respectively.
And 4, shielding unnecessary exciting light by using a light shield, and reserving useful exciting light.
Light of grade 1 of 532nm laser is selected as exciting light. The plus or minus 1-level exciting light passing through the light shielding plate is focused on a sample through the achromatic lens, two beams of light are overlapped in space and time to form interference fringes, slight pulse heating is generated in the space geometrical structure of an optical interference pattern, thermal expansion is generated, and then surface acoustic waves which are propagated in opposite directions are emitted, and wave vectors of the acoustic waves are determined by the intersection angle and the wavelength of the exciting light. Therefore, the included angle between the two excitation lights can be converted by converting different grating periods, and the wave vector of the emitted acoustic wave can be converted.
And 5, sequentially mounting the 830nm continuous laser and the spherical lens on the bottom plate. Wherein, the 830nm continuous laser light-emitting hole and the spherical lens are positioned on the same horizontal light path. The phase mask passes through the achromatic lens focus.
The height and angle of the light outlet hole of the 830nm laser and the height and position of the spherical lens are adjusted to ensure that the 830nm laser and the 532nm laser are overlapped on the grating.
The phase regulator is fixed in the optical path of the detection light, and the phase condition of the incident light can be accurately and dynamically regulated. The selected detection light and the reference light are 0-order and-2-order diffracted light of 830nm laser respectively, and the phase adjuster is used for adjusting the phase of the 830nm 0-order diffracted light. The 0-order diffraction light and the-2-order diffraction light after phase adjustment reach a phase matching condition, heterodyne phase optimization can be achieved, and finally the ultrasonic micro-vibration condition of nanometer magnitude can be detected. Taking the grating period of 4 μm, the selected detection light is 0-order diffraction light of 830nm laser, the focal length of the achromatic lens is 85mm as an example, and the vertical height of the center of the phase adjuster is adjusted to 48 mm.
Due to the size limitation 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, the horizontal light path where the 532nm pulse laser light outlet hole, the attenuation sheet, the cylindrical mirror and the phase mask plate are located, the light chopper, the achromatic lens and the phase adjuster are located, and 7 degrees is formed in the horizontal light path, the separation of exciting light and detecting light/reference light on the space is guaranteed at the angle, and therefore the phase adjuster is guaranteed not to shield light.
And 5, mounting the three-dimensional electric translation table. The three-position electric translation stage is horizontally arranged at an angle of 45 degrees with the optical path of the achromatic lens.
And 6, sequentially installing a focusing spherical lens and a photoelectric signal receiver in the signal receiving system. The focusing sphere is in the same horizontal optical path as the photoelectric signal receiver and is 90 degrees to the optical path of the achromatic lens.
And 7, starting a 532nm pulse light laser, an 830nm continuous laser, a three-dimensional electric translation table and a photoelectric signal detector, and measuring heterodyne signals under different grating periods.
Obtaining heterodyne signals after sample reflection and diffraction;
and calculating according to the heterodyne signals to obtain the thermal expansion rate of the sample.
In a further example, the specific method for obtaining the sample thermal expansion rate by calculating according to the heterodyne signal is as follows:
will ultrasonic displacement amplitude U0Substituting the expression of (3) and the expression of the acoustic wave length into the heterodyne signal to obtain the thermal diffusivity D of the film, wherein the ultrasonic displacement amplitude U0The expression is as follows:
wherein d is a linear thermal expansion coefficient, E0The average energy density of the exciting light is shown, rho is the material density, c is the specific heat, and D is the thermal diffusion coefficient;
the expression of the excited wave length q is as follows:
wherein theta is a diffraction angle obtained after the excitation light is diffracted by the phase mask plate, lambda is the wavelength of the excitation light, and lambda is a grating constant of the thermal grating generated by exciting the sample by the excitation light;
in a further example, the thermal diffusivity obtained is:
wherein τ is a relaxation time at the wavelength of the excited acoustic wave, and Λ is a relaxation time at the wavelength of the excited acoustic wave.
The detection principle of the invention is as follows:
532 pulse laser 1 launches the exciting light, behind attenuation piece 2, adjusts the exciting light energy, reaches cylindrical lens 3, focuses on the exciting light, focuses on phase mask 7. Before reaching 7, the excitation light passes through a dichroic mirror 4, which has no particular effect on the excitation light, but only allows it to pass through. After the excitation light reaches 7, the excitation light is divided into a plurality of diffracted laser beams, and only two excitation light beams are reserved after passing through a light shield 8. The two excitation lights pass through a first achromatic lens 9 and a second achromatic lens 11 and are focused on a sample 12 to form interference fringes, and finally, a signal is generated.
830 the continuous laser 5 emits detection light, which passes through the sphere lens 6 and is focused on the phase mask 7. Before reaching 7, the detection light is reflected by the dichroic mirror 4, and the reflected detection light is focused on 7. The detection light is divided into a plurality of diffraction laser beams after passing through the light chopper 7, and only two detection light beams are reserved after passing through the light chopper 8. Wherein 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. The probe light passes through the phase adjuster 10 and forms a phase difference with the reference light. The two final beams of light pass through a first achromatic lens 9 and a second achromatic lens 11, are focused on the sample, and are overlapped with a light spot formed by the excitation light. Finally, the detection light and the reference light carry the sample information excited by the excitation light, and the sample information reaches the detector 14 after being focused by the focusing spherical lens 13.
532nm pulse laser 1 in the photoacoustic excitation module emits pulse laser, reach the sample surface after a series of modulations, because the interference of light forms the fringe light source at the sample surface, thus arouse sample thermal grating, and based on the thermoelastic effect, can be under the condition of contactless sample, arouse surface acoustic wave in the sample, continuous laser that signal detection module emitted is coincided with excitation facula at the sample surface after a series of modulations, detect the acoustic information that light carried the sample surface and be received by signal receiving module, the signal received should be:
wherein ISFor detecting the intensity of light, I, after reflection by the sampleRIs the light intensity of the reference light after the diffraction of the sample, lambda is the wavelength of the excitation light, u (t) is the micro-vibration correlation function of the surface of the sample,
for the difference between the initial phases of the probe light and the reference light, the dynamic fine adjustment of the phase adjuster 10 in the heterodyne system is used to make
And since u (t) < lambda, the final signal received by the detector can be obtained as follows:
since the excitation light forms a fringe light source on the sample surface, the sample surface micro-vibration correlation function u (t) can be simplified to a sinusoidal form: u (t) ═ U0sin (qx), due to
The resulting heterodyne signal is therefore:
wherein I0Is the intensity of the reference light without diffraction by the sample, q is the wavelength of the excited sound wave, U0Is the amplitude of the ultrasonic displacement, due to ISRatio I0Is much larger, so that the final product isThe heterodyne signal I (t) is much larger than the signal obtained by the direct reference method, which is helpful for detecting the weaker vibration of the surface of the film,
ultrasonic displacement amplitude U0The expression is as follows:
wherein d is the coefficient of linear thermal expansion, E0The average energy density of the excitation light is rho is the material density, c is the specific heat and D is the thermal diffusion coefficient, so that the detected heterodyne signals I (t) and U are obtained0The relationship is as follows:
wherein, the formula of the excited wave length q is as follows:
wherein theta is a diffraction angle obtained after the excitation light is diffracted by the phase mask plate 7, lambda is the wavelength of the excitation light, and lambda is a grating constant of the thermal grating generated by exciting the sample by the excitation light.
Based on heterodyne signals I (t) and U0The relationship, yields a thermal diffusivity of:
the thermal diffusivity D of the film can be obtained by measuring the relaxation time under a certain excited sound wave wavelength, the excited sound wave wavelength can be changed by adjusting the grating period of the phase mask 7, namely, the grating constant of the vibration grating caused by the sound wave is changed, and the correctness of the calculated thermal diffusivity of the film is verified by measuring the relaxation time under different sound wave wavelengths.
Fig. 3 is a diagram illustrating simulation of focusing spots of the excitation light and the detection light/reference light at the final sample. By adjusting the positions of the two achromatic lenses, the two beams of light of the exciting light and the two beams of light of the detecting light are ensured to be overlapped at the same position of the sample, and the requirements on the size and the intensity of a light spot are met.
FIG. 4 is a schematic diagram showing the reflection and diffraction of probe light and excitation light, respectively, after passing through a thin film excited by the excitation light. The detection light and the reference light are reflected and diffracted by the micro-vibration sample respectively, and finally reach the photoelectric signal receiver through a focusing spherical mirror in the signal receiving module. And finally, the receiver receives a heterodyne signal after the detection light is coupled with the reference light. And obtaining an optimal signal through dynamic adjustment of a phase adjuster in the heterodyne system, and finally demodulating the thermal diffusivity of the nano film sample.
The invention can conveniently and quickly excite sound waves with different wavelengths to the sample in the same system, realize the acquisition of a plurality of groups of data and ensure that the experimental result is more accurate. And the heterodyne detection method used in the excitation and detection module is optimized, so that the detection is more convenient and accurate under the condition that each optical element does not shield the light path.
Claims (10)
1. The laser ultrasonic system for detecting the thermal diffusivity of the nano-film in a non-contact manner is characterized by comprising an excitation light emitting module, a detection light emitting module, a dichroic mirror (4), a phase mask plate (7), a light chopper (8), a first achromatic lens (9), a phase adjuster (10), a second achromatic lens (11), a mobile sample stage and a signal receiving module, wherein the laser ultrasonic system is set as follows:
excitation light emitted by the excitation light emitting module is transmitted by the dichroic mirror (4) and then focused on the phase mask plate (7), the phase mask plate (7) divides the excitation light into two beams, and the two beams of excitation light are sequentially focused on the surface of a sample arranged on the movable sample stage through the light chopper (8), the first achromatic lens (9) and the second achromatic lens (11);
the detection light emitted by the detection light emitting module is reflected by the dichroic mirror (4) and then focused on the phase mask plate (7), the phase mask plate (7) divides the detection light into two beams, one beam of the detection light is used as detection light and sequentially focused on the surface of a sample arranged on the movable sample stage through the light chopper (8), the first achromatic lens (9), the phase adjuster (10) and the second achromatic lens (11), and the other beam of the detection light is used as reference light and sequentially focused on the surface of the sample arranged on the movable sample stage through the light chopper (8), the first achromatic lens (9) and the second achromatic lens (11);
the two excitation lights are overlapped with the detection light and the reference light at the same position of the sample;
the signal receiving module is used for receiving heterodyne signals after the reflection and the diffraction of the sample.
2. The laser ultrasonic system for detecting the thermal diffusivity of a nano-film in a non-contact manner according to claim 1, wherein the light emitting module comprises a 532nm pulse laser (1), an attenuation sheet (2) and a cylindrical mirror (3), the 532nm pulse laser (1), the attenuation sheet (2) and the cylindrical mirror (3) are located on the same optical axis, and the cylindrical mirror (3) focuses the excitation light onto a phase mask (7).
3. The laser ultrasonic system for non-contact detection of thermal diffusivity of nano thin film according to claim 1, wherein the detection light emitting module comprises an 830nm continuous laser (5) and a spherical lens (6), and the 830nm continuous laser (5) and the spherical lens (6) are located on the same optical axis.
4. The laser ultrasonic system for non-contact detection of thermal diffusivity of a nano-film according to claim 1, characterized in that the signal receiving module comprises a focusing sphere lens (13) and a photoelectric signal receiver (14):
the focusing sphere lens (13) is used for coupling the detection light carrying the sample information excited by the excitation light and the reference light into the photoelectric detector (14) to obtain a heterodyne signal.
5. The laser ultrasonic system for non-contact detection of thermal diffusivity of nano-film according to claim 1, characterized in that the detection light reflected by the dichroic mirror (4) and the excitation light transmitted by the dichroic mirror (4) converge on 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 through without blocking the excitation light.
6. The laser ultrasonic system for non-contact detection of thermal diffusivity of a nano-film according to claim 1, wherein heterodyne signals received by the signal receiving module are:
wherein q is the wavelength of the excited sound wave, ISFor detecting the intensity of light, I, after reflection by the sampleRLambda is the wavelength of the excitation light, and u (t) is the micro-vibration correlation function of the sample surface.
7. The laser ultrasonic system for non-contact detection of thermal diffusivity of a nano-film according to claim 6, wherein the sample surface micro-vibration correlation function is specifically as follows:
u(t)＝U0sin(qx)
in the formula of U0Is the amplitude of the ultrasonic displacement, q is the wavelength of the excited sound wave;
the reference light intensity after sample diffraction is specifically as follows:
in the formula I0The intensity of the reference light without diffraction by the sample.
8. A method for detecting thermal diffusivity of a nano-film in a non-contact manner is characterized by comprising the following steps:
constructing the laser ultrasound system of any one of claims 1 to 7;
obtaining heterodyne signals after sample reflection and diffraction;
and calculating according to the heterodyne signals to obtain the thermal diffusivity of the sample.
9. The method for detecting the thermal diffusivity of a nano-film in a non-contact manner according to claim 8, wherein the specific method for obtaining the thermal diffusivity of a sample by solving according to heterodyne signals is as follows:
will ultrasonic displacement amplitude U0Substituting the expression of (3) and the expression of the acoustic wave length into the heterodyne signal to obtain the thermal diffusivity D of the film, wherein the ultrasonic displacement amplitude U0The expression is as follows:
wherein d is a linear thermal expansion coefficient, E0The average energy density of the exciting light is shown, rho is the material density, c is the specific heat, and D is the thermal diffusion coefficient;
the expression of the excited wave length q is as follows:
wherein theta is a diffraction angle obtained after the excitation light is diffracted by the phase mask plate (7), lambda is the wavelength of the excitation light, and lambda is a grating constant of the thermal grating generated by exciting the sample by the excitation light.
10. The method for detecting the thermal diffusivity of a nano-film in a non-contact manner as claimed in claim 8, wherein the obtained thermal diffusivity is as follows:
wherein τ is a relaxation time at the wavelength of the excited acoustic wave, and Λ is a relaxation time at the wavelength of the excited acoustic wave.
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