Broadband frequency domain heat reflection method for measuring thermophysical property of solid material

Abstract

The invention discloses a broadband frequency domain heat reflection method for measuring the thermophysical property of a solid material, which comprises the following steps: carrying out intensity modulation on pump light, irradiating the pump light to the surface of a sample to heat the sample, reflecting probe light after reaching the surface of the sample, and generating a difference frequency signal on the surface of the sample or in front of a photoelectric detector; and measuring phases of the pump light, the probe light and the frequency mixing light by using a lock-in amplifier, calculating a thermal phase, and fitting the thermal phase to obtain required thermophysical parameters. The invention provides an innovative difference frequency method for solving the problem of low signal-to-noise ratio of a frequency domain heat reflection method under high-frequency measurement, the method can effectively improve the signal-to-noise ratio when the measurement frequency is greater than 20MHz, and finally, the heating frequency as high as 200MHz is realized. And more data points can be obtained by a wider heating frequency interval, so that a proper interval can be selected according to a sensitivity curve to carry out parameter fitting, and the accuracy of thermophysical property measurement is improved.

Description

Technical Field

[0001] The invention relates to a broadband frequency domain thermal reflection method, in particular to a broadband frequency domain thermal reflection method for measuring thermal properties of solid materials. Background Art

[0002] Frequency domain thermoreflectometry is an advanced thermophysical property measurement technique. It uses a periodically modulated continuous laser as a heat source to heat the sample, and the temperature of the sample surface changes periodically. When the temperature change is kept within a small range, there is a linear relationship between the light intensity reflection coefficient and the temperature. Using this feature, we can use another continuous laser as a detection light source to illuminate the sample, and measure the amplitude and phase of the surface temperature change by analyzing the light reflected from the sample surface. The phase difference between the sample surface temperature and the periodic heat source heating is closely related to the thermophysical parameters of the sample. By measuring the phase difference between the two at different heating frequencies, and then using the inverse analysis method, the thermophysical parameters of the sample can be determined, including thermal conductivity, interface thermal conductivity and volume heat capacity. This method not only provides accurate measurement results, but also is easy to operate, which brings great convenience to the fields of materials science and thermophysical property research.

[0003] However, a relatively wide heating frequency range is often required in experimental measurements, because the accuracy of the thermophysical parameters obtained by reverse analysis depends on the sensitivity. Since the sensitivity of each parameter in different heating frequency ranges varies greatly, the accuracy of reverse analysis also varies greatly accordingly. Therefore, if the parameters of the measured material need to be accurately obtained, a wide heating frequency range measurement is required, so that the measurement data in the heating frequency range with high sensitivity corresponding to each parameter can be selected for accurate reverse analysis.

[0004] In the traditional frequency domain thermoreflectometry, an electro-optic modulator (EOM) is used to modulate the intensity of the pump light, and a sinusoidal electrical signal is introduced into the pump light path to generate a modulated optical signal. The heating frequency of the traditional method is usually below 20MHz because the frequency response signal of the detection light is Among them, C TR is the light reflection coefficient of the structure surface; Q0 and Q1 are the power of the pump light and the detection light; k is the Hankel transformation; C and D are the matrix elements in the heat conduction matrix; ω0 and ω1 are the radii of the heating light and the detection light. The intensity of the detection light signal is positively correlated with the amplitude R, which is This formula shows that when the heating frequency increases, the amplitude of the signal decreases. In addition, the high-frequency region is accompanied by an increase in coherent noise and environmental noise. Specifically, when two cables transmit high-frequency signals, especially signals above 20MHz, the main source of noise is the cable connecting the EOM amplifier and the EOM. The high-voltage signals on these cables will produce a broadcast effect at the same frequency as the thermal signal, which will be received by the cable connecting the photodetector and the phase-locked amplifier. Since this cable acts as an antenna in this case, the noise signal it receives will be superimposed on the thermal signal, further reducing the accuracy of the measurement. In short, these factors together cause the signal-to-noise ratio of the frequency domain thermoreflectometry method to be significantly reduced at higher heating frequencies. Summary of the invention

[0005] Purpose of the invention: The present invention aims to provide a broadband frequency-domain thermal reflection method for measuring the thermal properties of solid materials, so as to solve the problem of low signal-to-noise ratio of the frequency-domain thermal reflection method under high-frequency measurement. At the same time, a wider heating frequency range can obtain more data points, and then a suitable interval can be selected according to its sensitivity curve for parameter fitting, thereby improving the accuracy of thermal property measurement.

[0006] Technical solution: The broadband frequency-domain thermal reflection method for measuring the thermal properties of solid materials described in the present invention can select the difference frequency to occur on the sample surface or in front of the photoelectric detector.

[0007] When the difference frequency occurs on the sample surface, the method includes: the pump laser and the detection laser are synchronously intensity modulated, and the signal generator can output multiple frequencies, including the difference frequency signal, for use by the phase-locked amplifier as a reference frequency. The pump light is irradiated onto the sample surface, causing the sample to heat up. At the same time, the signal carried by the detection light meets the signal carried by the pump light on the sample surface and mixes. The two beams of light are then reflected back to the photodetector together. The phase of the pump light, the detection light, and the mixed light is measured using the phase-locked amplifier, and the thermal phase is calculated. The thermal phase is fitted to obtain the required thermal physical property parameters. The calculation formula for the thermal phase is as follows:

[0008]

[0009] in, is the phase of the pump light, To detect the phase of light, is the phase of the mixed light.

[0010] Furthermore, the pump light laser and the probe light laser are independently and differently intensity modulated by respective signal generators and electro-optical modulators (EOMs), at frequencies f1 and f2, respectively.

[0011] When the difference frequency occurs in front of the photodetector, the method includes: the electro-optic modulator applies intensity modulation of the pump light with a frequency of f1, the pump light irradiates the sample surface, causes the sample to heat, and makes the sample surface temperature change periodically, and then after the unmodulated detection light reaches the sample surface, due to the periodic reflectivity change caused by the temperature change, the detection light carries a signal with a frequency of f1 and a thermal phase after being reflected by the sample surface, and this signal is then further modulated by the electro-optic modulator in front of the photodetector, and after passing through the low-pass filter, the signal frequency is converted to f1-f2, and finally captured by the phase-locked amplifier. The phase of the pump light, the detection light and the mixed light is measured by the phase-locked amplifier, and the thermal phase is calculated, and the required thermal physical property parameters are obtained after fitting the thermal phase. The calculation formula of the thermal phase is the same as above.

[0012] Furthermore, two signal generators are connected to the mixer, and a reference signal with a frequency of f1-f2 is generated by the mixer and supplied to the lock-in amplifier.

[0013] Furthermore, the frequency difference Δf during mixing is 100KHz-50MHz. The present invention maintains the difference Δf of the two modulation frequencies at a relatively low fixed value, and this difference is set as the reference frequency of the phase-locked amplifier. The advantage of this design is that the frequency difference can be directly performed at the sample interface or in front of the photodetector, thereby avoiding the influence of coherent noise in the traditional technical solution, thereby improving the efficiency of signal processing and the accuracy of measurement.

[0014] Furthermore, a beam splitter is provided on the pump light path, a long-wave pass dichroic mirror is provided on the detection light path, and filters corresponding to the two kinds of light are installed in front of the photodetector.

[0015] Furthermore, the thermophysical property parameters include thermal conductivity, volume heat capacity and interface thermal conductance.

[0016] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages: the method can effectively improve the signal-to-noise ratio when the measurement frequency is greater than 20 MHz, and ultimately achieve a heating frequency of up to 200 MHz. A wider heating frequency range can obtain more data points, and then the appropriate interval can be selected according to its sensitivity curve for parameter fitting, thereby improving the accuracy of thermal property measurement. In addition, the present invention can realize the difference frequency signal measurement of the reflected detection light under high pump light modulation frequency, and maintain an excellent signal-to-noise ratio. A higher heating frequency allows more data points to be captured under high-frequency modulation conditions. The increase in these data points provides a wider range of choices to select a suitable interval for parameter fitting according to the sensitivity curve. This scheme significantly improves the accuracy and reliability of thermal property measurements, and lays a solid foundation for the accurate analysis of experimental data. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 This is a diagram of the optical path design structure of Example 1;

[0018] Figure 2 This is a comparison chart of the signal-to-noise ratios obtained by testing the present invention and the traditional method;

[0019] Figure 3 is the thermal phase fitting diagram;

[0020] Figure 4 This is a sensitivity curve diagram of Example 1;

[0021] Figure 5 This is the optical path design structure diagram of Example 2. DETAILED DESCRIPTION

[0022] The technical solution of the present invention is further described below in conjunction with the accompanying drawings.

[0023] Figure 1 and Figure 5 The reference numerals in the figures are: 1 - signal generator, 2 - electro-optic modulator, 3 - phase-locked amplifier, 4 - photodetector, 5 - mixer.

[0024] Example 1

[0025] This embodiment uses a pump laser and a detection laser (pump light 400nm, detection light 800nm) to perform intensity modulation through a signal generator 1 and an EOM. The signal generator 1 can output multiple frequencies, including difference frequency signals, for use as reference frequencies by a phase-locked amplifier 3. A beam splitter is provided on the pump light path, and a long-wave pass dichroic mirror is provided on the detection light path. The pump light irradiates the sample surface, causing the sample to heat up. At the same time, the signal carried by the detection light meets the signal carried by the pump light on the sample surface and mixes. The two beams of light are then reflected back to the photodetector 4 together. In order to accurately separate and detect the two lights, two corresponding light filters are installed in front of the photodetector 4. The phase-locked amplifier 3 can lock multiple frequencies at the same time and perform phase measurement in a specific frequency range. In this way, the phases of the pump light, the detection light, and the mixed detection light are measured respectively. Based on these measurement results, the thermal phase can be calculated. Finally, the thermal phase is fitted in combination with the heat transfer model to obtain the required thermal physical property parameters.

[0026] Schematic diagram of the signal that the pump light and the probe light carry frequencies f1 and f2 respectively after being modulated by the signal generator 1 and the EOM. Since ω=2πf, the pump light is Y1=A1sin(ω1t) and the probe light is Y2=A2sin(ω2t).

[0027] When the two beams of light reach the sample surface, the pump light generates heat on the sample surface, and the heat propagates in the sample, causing a phase delay caused by the surface temperature change. It contains information closely related to the thermal properties of the sample. Under the condition of a small temperature rise, the change in light reflectivity on the sample surface maintains a linear relationship with the temperature change. Due to the heating effect of the pump light, the surface temperature will show periodic changes following the heating frequency. This periodic change will also affect the reflection characteristics of the probe light. In addition, since the probe light and the pump light have undergone different instrument paths and reflection processes, they each carry an additional phase and Then during reflection, where Y 混 It represents the mixed light in the reflected detection light, which is obtained by mathematical calculation of the reflectivity change caused by the detection light and the surface temperature change. The high-frequency (ω1+ω2) signal is omitted here because it is not the signal we need and will be filtered out by the phase-locked amplifier.

[0028] When the two beams of light reach the photodetector, they will also carry additional phases due to the different optical paths they have experienced. and Mixed light carries additional phase but because The distance from the sample surface to the photodetector is the same for the three beams, so By replacing the corresponding filter in front of the photodetector, the pump light and the probe light can be passed separately, so that their phases can be measured separately. In particular, the difference frequency reflected probe light signal is used here, and its frequency is significantly lower than the modulation frequency of the pump light. This method effectively reduces the coherent noise, thereby significantly improving the signal-to-noise ratio, such as Figure 2 As shown, the yellow curve is the noise measured after the mixing of the present invention, and the black curve is the noise measured by the conventional method (reference: Schmidt AJ, Cheaito R, Chiesa MA frequency-domain thermoreflectance method for the characterization of thermal properties [J]. Review of Scientific Instruments, 2009, 80 (9): 617.).

[0029] Based on this, we can get By fitting the thermal conductivity model, thermal physical parameters such as thermal conductivity and interface thermal conductivity can be obtained.

[0030] Fitting process: This experiment mainly measures the thermal phase angle The theoretical calculation formula is as follows. The lsqcurvefit model is used in MATLAB to fit. By setting 1-3 unknown parameters to be fitted, the remaining parameters are substituted into the literature values ​​or actual measured values, and the fitting range of each parameter is set as the initial value according to the actual situation, as shown in Table 1 and Figure 3 As shown in the figure, the sensitivity curve is as follows Figure 4 shown.

[0031] The frequency response function is

[0032] Thermal phase angle

[0033] Table 1

[0034]

[0035] *: Thermophysical property parameters obtained by fitting

[0036] Example 2

[0037] This embodiment uses an electro-optic modulator 2 to modulate the intensity of the pump light with a frequency of f1, and then the detection light carries a signal with a frequency of f1 after being reflected by the sample surface. This signal is then further modulated by the EOM in front of the photodetector 4, and after passing through a low-pass filter, the signal frequency is converted to f1-f2, and finally captured by the phase-locked amplifier 3. At the same time, the two signal generators 1 generate a reference signal with a frequency of f1-f2 through the mixer 5, which is supplied to the phase-locked amplifier 3. See the optical path design structure diagram Figure 5 , The calculation method and the fitting process of the thermal phase are similar to those in Example 1.

Claims

1. A broadband frequency domain thermal reflection method for measuring the thermal properties of solid materials, characterized in that: The method comprises: intensity modulating the pump light, irradiating the pump light to the sample surface to cause the sample to heat up, reflecting the probe light after reaching the sample surface, and generating a difference frequency signal on the sample surface or in front of a photodetector, measuring the phases of the pump light, the probe light and the mixed light by a lock-in amplifier, and calculating the thermal phase, fitting the thermal phase to obtain the required thermal physical property parameters; the calculation formula of the thermal phase is as follows: in, is the phase of the pump light, To detect the phase of light, is the phase of the mixed light.

2. The broadband frequency domain thermal reflection method for measuring the thermal properties of solid materials according to claim 1, characterized in that: When a difference frequency signal is generated on the sample surface, the pump light and the detection light are intensity modulated synchronously. The signal carried by the detection light and the signal carried by the pump light meet and mix on the sample surface at the same time. The two beams of light are then reflected back to the photodetector together.

3. The broadband frequency domain thermal reflection method for measuring the thermal properties of solid materials according to claim 1, characterized in that: In the case where a difference frequency signal is generated in front of the photodetector, the detection light is not modulated, is reflected after reaching the sample surface, and is mixed after passing through the electro-optic modulator in front of the photodetector.

4. The broadband frequency domain thermal reflection method for measuring the thermal properties of solid materials according to claim 2, characterized in that: The pump laser and the detection laser are synchronously intensity modulated by a signal generator and / or an electro-optic modulator.

5. The broadband frequency domain thermal reflection method for measuring the thermal properties of solid materials according to claim 3, characterized in that: An electro-optic modulator is connected in front of the photodetector and the detection laser. The electro-optic modulator is connected to a signal generator. The two signal generators are connected to a mixer. The mixer generates a reference signal and supplies it to a phase-locked amplifier.

6. The broadband frequency domain thermal reflection method for measuring the thermal properties of solid materials according to claim 2 or 3, characterized in that: The frequency difference Δf during mixing is 100KHz-50MHz.

7. The broadband frequency domain thermal reflection method for measuring the thermal properties of solid materials according to claim 1, characterized in that: A beam splitter is provided on the pump optical path, and a long-wavelength dichroic mirror is provided on the detection optical path.

8. The broadband frequency domain thermal reflection method for measuring the thermal properties of solid materials according to claim 1, characterized in that: Filters corresponding to the two types of light are installed in front of the photodetector.

9. The broadband frequency domain thermal reflection method for measuring the thermal properties of solid materials according to claim 1, characterized in that: This method is applicable to heating frequencies up to 200 MHz.

10. The broadband frequency domain thermal reflection method for measuring the thermal properties of solid materials according to claim 1, characterized in that: The thermophysical property parameters include thermal conductivity, volume heat capacity and interface thermal conductance.

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